Closed absorption cycles use heat to produce at least one of refrigeration, heat pumping, and power. Especially with the ammonia-water type absorption cycle, but also generally with all types, the absorption step is recognized as being the most difficult of the four major heat and mass transfer steps of the cycle (absorption, desorption, condensation, and evaporation). With ammonia-water absorption, the water vapor absorbs preferentially into the absorbent solution relative to the ammonia vapor, leaving higher purity ammonia vapor at the liquid interface. This reduces the concentration driving force, thus impeding mass transfer of vapor into liquid. It is hypothesized that the cause of this is that the flux of vapor into the liquid has a much higher NH3 concentration than the liquid does, thus establishing a concentration gradient in the liquid, that feeds back to a concentration gradient in the vapor.
Ammonia-water absorption has conventionally been accomplished industrially by spraying absorbent solution over a tube bundle (or into the tubes) in the presence of the vapor to be absorbed, with cooling water on the other side of the tubes. An example of each is found in Bogart 1981 and in Malewski 1986. In residential-scale air-cooled units, the solution and vapor are apportioned into multiple parallel fin-tubes, with air blown across the fins. Pairs of tubes are joined after about two thirds of the absorption is accomplished, to keep the tube-side velocity in a desirable range for vapor-liquid mixing. See for example U.S. Pat. No. 3,580,001.
More recently, shell and coil absorbers have been used to improve the ammonia-water absorption rates. For example, U.S. Pat. No. 6,679,083 discloses cocurrent upflow of ammonia-water liquid and vapor on the shell side of this geometry, with countercurrent downflow of coolant on the tube side. With all three of the above approaches to absorption, the problem persists that as vapor absorption proceeds, the vapor volume decreases, the fluid velocities become lower and more quiescent, and hence absorption slows markedly. As a result large temperature driving forces become necessary, some of the cooling is wasted on subcooling the absorbent liquid, and large, costly absorbers are required.
With LiBr absorption (and other water vapor absorbents such as hydroxide mixtures), the vapor phase is single component (pure water vapor), since the absorbent is non-volatile. However absorption is still the limiting step, for a different reason=the very low pressure (deep vacuum). Trace amounts of inert gas will blanket the tubes at one hundredth of an atmosphere. LiBr absorption is frequently accomplished by spraying absorbent solution on cooled tube bundles, similar to some ammonia-water applications. This is referred to as falling film absorption. There has been some investigation reported on an alternative technique for LiBr absorption—adiabatic spray. Instead of spraying the solution onto a tube bundle, it is sprayed into a vacuum chamber, so the vapor is absorbed adiabatically. All of the heat of absorption goes into sensible heating of the solution, so it becomes quite warm. In order to limit the temperature rise to practical values, a very large volume spray is used, and the spray liquid is continuously recirculated by pumping through a liquid cooler and back to the spray nozzles. The advantage of this technique is that the heat transfer step enjoys very large transfer coefficients characteristic of liquid-liquid heat transfer. The disadvantage is that the recirculating liquid flowrate must be five to ten times larger than the flowrate of solution between absorber and desorber. This technique is described in Warnakulasuriya, F. and Worek, W. M., 2008 (Int J. Heat Mass Transf) vol 51; and in Summerer, F., et al, “Hydroxide Absorption Heat Pumps with Spray Absorber”, ASHRAE Paper AT-96-17-5, Transactions vol 102, Part 1, 1996. Also of interest are: Ryan, W. A., “Water Absorption in an Adiabatic Spray of Aqueous Lithium Bromide Solution”, AES Vol 31, ASME 1993; and Venegas, M., et al, “Spray Absorbers in Absorption Systems Using Lithium Nitrate-Ammonia Solution”, International Journal of Refrigeration, 2005, vol 28; and Gutierrez, G., et al, 2007, “Performance Analysis of an adiabatic absorption test rig fed with a low temperature heat source”, Tarragona.
Thus included among the problems of the prior art absorbers is that the recirculated spray adiabatic absorbers require exceptionally large pumping rates of absorbent solution; that LiBr falling film absorbers have pressure drop limitations and inert gas accumulation limitations; and that ammonia=water absorbers have performance limitations associated with low vapor velocities at the end of the absorption step, and also with buildup of excess ammonia concentration at the vapor-liquid interface under quiescent (low velocity) conditions.